Who are the Major Producers Of Methylmercury in the Environment?
The gene pair hgcAB is essential for microbial mercury methylation. Therefore, improved techniques for capturing hgcAB presence and diversity are necessary for identifying both the major players in environmental methylmercury production as well as the evolutionary history of the gene pair. My research has improved molecular and meta-omic methods for using the genes as biomarkers for identifying key producers of the toxin in terrestrial, freshwater, and marine ecosystems. By applying these tools to environmental samples, our understanding of mercury methylator abundance and diversity, and their potential for methylmercury generation has grown significantly since the gene pair’s discovery in 2013.
Integrative Omics Approach to Predicting Mercury Biogeochemistry
Microbial Hg transformations under changing redox are an ideal test system for connecting molecular mechanisms to ecosystem-level impacts. This is in part because the genetics of microbial Hg transformations are well-defined and because the mechanisms are redox-regulated and tightly coupled to nitrogen, iron, sulfur, and carbon dynamics. Aquatic ecosystems under disturbance provide the opportunity to investigate the response of the microbiome to shifts in redox, carbon, and nutrient dynamics and impact on mercury cycling. In my research, I integrate various meta-omic (i.e. metagenomic, metatranscriptomic, metaproteomic, metabolomic) and molecular approaches with geochemical analyses to measure microbial response and ecosystem-level changes to toxic metal cycling following a disturbance.
Biochemical Mechanism of Mercury Methylation
Often environmental measurements of Hg-methylator abundance do not positively correlate with methylmercury (MeHg) concentrations (top left; Christensen et al. 2019). Therefore, abundance of Hg-methylating organisms alone cannot predict net MeHg production. Several microbial and geochemical parameters influence environmental MeHg production and accumulation. One significant hurdle to constraining these parameters is understanding the biochemical mechanism of Hg methylation (bottom left; Parks et al. 2013). During my postdoctoral tenure at Oak Ridge National Laboratory, I conducted a multi-omics exploration into the biochemical function of Hg methylation proteins in model organism, Desulfovibrio desulfuricans ND132. By taking a systems biology approach we have been able to get a holistic view of how Hg methylation is connected to cellular metabolism. This helps define the factors that drive activity of Hg methylating organisms, improving our ability to predict how changes to microbial community dynamics will impact environmental cycling of the toxin.
This work was funded by the DOE Environmental Molecular Sciences Laboratory (EMSL) (user proposal 50174) and the ORNL Mercury SFA sponsored by Subsurface Biogeochemical Research (SBR) program, U.S. Department of Energy’s Office of Biological and Environmental Research.
Effects of Environmental Perturbations on Microbial Community Dynamics
At ORNL, we utilized in-field groundwater bioreactors to temporally monitor how alterations in abiotic forces (i.e. contamination, nutrient and carbon availability) affect groundwater microbial dynamics. Groundwater across Oak Ridge (TN, USA) contains a gradient of contaminants generated during the research and production of nuclear materials, providing a field laboratory for studying how subsurface microorganisms shape and are shaped by their environment’s geochemistry. This research demonstrates how these geochemical gradients shift subsurface microbial community composition, limiting microbial diversity and selecting for microbial metabolisms capable of utilizing available redox-sensitive elements (i.e. U, Fe, NO3) for energy.
This work was part of ENIGMA- Ecosystems and Networks Integrated with Genes and Molecular Assemblies, a Scientific Focus Area Program supported by the Genomic Sciences Program in the U.S. Department of Energy’s Office of Biological and Environmental Research.